In a manufacturing step for a hot galvanized and alloyed band steel, a band steel is generally subjected to a hot galvanizing bath to deposit a hot galvanized layer on the surface of the band steel, and the amount of plated deposition is reduced to a target value by blowing a gas to the surface of the band steel, followed by passing the band steel through an alloying furnace. The diffusion occurs as a result of a heat treatment within the furnace, whereby the plated layer is converted into an alloy of iron and zinc. Excellent flaking and powdering resistances are requisite important qualities for the hot galvanized and alloyed band steel which is manufactured in this manner. To obtain a hot galvanized and alloyed band steel having a preferable quality, it is necessary to control the temperature of the alloying furnace or the conveying speed of the band steel during the manufacturing step so that the degree of alloying, which may be represented by the percentage of iron content in the plated layer, for example, may be controlled to a given condition, thus preventing the occurrence of underalloying or overalloying.
For example, Japanese Laid-Open Patent Application No. 279,738/1989 discloses a manufacturing process in which the flaking resistance can be improved by specifying initial heat treatment conditions for the alloying treatment. Also Japanese Laid-Open Patent Application No. 252,761/1989 discloses a feedback control in which the degree of burning a burner or the magnitude of the heat input is regulated in accordance with a deviation between a measured plate temperature and a target plate temperature which is established on the basis of the conveying speed of the sheet steel, the amount of zinc deposited and Al concentration in the plating bath.
First Task:
It is possible to estimate the quality, in particular, the degree of alloying of a hot galvanized and alloyed band steel from process parameters, in particular, from the plate temperature of the band steel, or to estimate the degree of alloying from a measurement of the emissivity or reflectivity of the band steel. However, an alloying process is greatly complicated, and as a consequence, if the heat input is controlled so that the plate temperature approaches a predetermined target value or if the heat input is controlled so that the emissivity or reflectivity approaches its target value, the degree of alloying attained may sometimes miss the target value. When the degree of alloying is wanting, an unalloyed surface is likely to occur, causing a degradation in the quality of the hot galvanized and alloyed band steel.
Accordingly, it is a first task of the invention to obtain a hot galvanized and alloyed band steel of high quality even in actual operations involving large variations in the steel variety, conveying speed, plated depositions or the like by maintaining a proper control of the heat input to prevent the occurrence of unalloyed surfaces.
Second Task:
When performing a feedback control in which the heat input is compensated for in accordance with the deviation between a detected value and a target value by utilizing a plate thermometer to determine the temperature of the band steel, the plate thermometer must be located at a spaced location from the burner, so that in the event a change occurs in the temperature of the band steel, a time lag is caused until such change is actually detected by the plate thermometer. If such a temperature change is of an increased magnitude, a resulting error in the heat input being controlled which is caused by such time lag will result in the occurrence of a region of underalloying (unalloyed surface) or overalloying produced on the band steel, thus resulting in a reduced yield. In an actual manufacturing step, a number of steel coils are joined together, by soldering end to end, into a single band steel, which is then subjected into a continuous alloying treatment of hot galvanized band steel. However, in the region of joints between steel coils, operating parameters such as the steel variety, plate thickness, plate width, conveying speed, plated deposition or the like often change, and accordingly when such region is being treated, the time lag which is caused during the feedback control may result in a region of underalloying or overalloying being produced on the band steel.
It is to be understood that during a hot rolling and a cooling step which precedes the plating step, a leading and a trailing portion of the band steel is likely to be cooled more strongly than the remainder, and thus produce a differential effect of the cooling process which is dependent upon the location. This results in a differential or non-uniform composition of the rolled steel band. Accordingly, a cooling process called U-pattern cooling may be employed to provide a uniform composition of the band steel. Specifically, the degree of cooling applied to the leading and the trailing portion of the coil is reduced as compared with the remainder. A steel material which is subject to such U-pattern cooling (which is referred to as U-pattern material) presents a problem during the heat treatment of the hot galvanized steel in the alloying furnace in that the leading and the trailing portion are less subject to such heat treatment, and is likely to cause the occurrence of unalloyed surfaces. Such differential degree of treatment dependent on the location is not so remarkable for a steel material which is manufactured according to a normal cooling process. As a consequence, during the alloying treatment of U-pattern material, a conventional feedback control results in a region of insufficient treatment or overalloying and a consequent reduction in the yield, due to a time lag in the compensation of the heat input for the leading and the trailing end of the steel material.
Accordingly, it is a second task of the invention to enhance the yield of a hot galvanized and alloyed band steel by maintaining a proper control of the heat input so as to prevent the occurrence of an underalloying or overalloying in the actual operations involving large variations in the steel variety, conveying speed, plating deposition or the like and even when a band steel such as U-pattern material which is subject to a special cooling treatment is being treated.
Third Task:
It is possible to estimate the quality, in particular, the degree of alloying of a hot galvanized and alloyed band steel from process parameters, for example, from the plate temperature of the band steel. However, the complicated nature of the alloying process may cause the degree of alloying to miss its target value due to various factors even when the heat input is controlled so that the plate temperature approaches a predetermined target value. In particular, when the band steel changes from a variety which is amenable to alloying to another variety which is less amenable to alloying, an unalloyed surface is likely to occur to a substantial degree. When underalloying results in producing an unalloyed surface, the quality of the hot galvanized and alloyed band steel will be greatly degraded, thus reducing the yield.
Accordingly, it is a third task of the invention to obtain a hot galvanized and alloyed band steel of a high quality by maintaining a proper control of the heat input so as to prevent the occurrence of unalloyed surfaces in actual operations involving large variations in the steel variety, conveying speed, plating deposition or the like.
Fourth Task:
The degree of alloying of a band steel has a significant correlation with the optical reflectivity or emissivity of the band surface, and hence it is possible to determine the actual degree of alloying of the band steel from a measurement of the optical reflectivity or emissivity. Theoretically, it is possible to accurately compensate for the heat input by utilizing a feedback of such result to the control of the heat input.
However, in practice, the sensitivity of detecting an overalloying in accordance with a measurement of an optical reflectivity is substantially low even though the sensitivity of detecting the occurrence of an unalloyed surface in accordance with a measurement of the optical reflectivity or the like is relatively good. As a consequence, when attempting a control in which the heat input is increased in response to the occurrence of an unalloyed surface and is decreased in response to the detection of an overalloying by detecting unalloyed surfaces and overalloying on the basis of the measured optical reflectivity or emissivity, the resulting actual heat input tends to shift toward the overalloying (or excessive heat input) beyond the optimum heat input, with consequence that the powdering resistance of the band steel is degraded to reduce the product yield.
Accordingly, it is a fourth task of the invention to control the heat input to an alloying furnace so as to bring the degree of alloying of a hot galvanized and alloyed band steel to an optimum condition, thereby improving the yield of such band steel.
Fifth Task:
In an investigation conducted by the inventors, it is found that during the control of an alloying of a hot galvanized and alloyed band steel containing 6 to 13% of iron content in the plated alloy, the suppression of formation of .zeta. phase or the like in the surface of the plated layer is effective in improving the flaking resistance while when the band steel is passed through a heating zone and then through an insulated heated zone for purpose of achieving uniform alloying, a control on the basis of the heat input to the heating zone which is calculated on the basis of the steel variety, conveying speed, the plating depositions or the like is effective for the intended purpose.
However, the alloying process is complicated and non-linear. As a consequence, in actual operations involving large variations in the steel variety, conveying speed, the plating deposition or the like, a single formula cannot be relied on to provide a proper amount of heat input continuously. While a plurality of formulae may be provided which may be selectively used in accordance with the actual value of these parameters, it is very difficult to render a decision in choosing one of these formulae for a specified range of parameters in order to provide a proper heat input. In particular, for a boundary between the ranges across which a selected formula is changed, the proper heat input cannot be determined using either formula.
Accordingly, it is a fifth task of the invention to maintain a proper heat input in actual operations involving large variations in the steel variety, conveying speed, plating deposition or the like.